Recently there has been much research into different devices and methods for conserving the energy of rechargeable batteries. The above-referenced patent discloses a discharge level monitor for batteries wherein the current is passed through a sensing resistor and the voltage across the resistor is amplified and integrated over time and the result of the integration is stored in a capacitor discharged by a switch whenever a threshold voltage is achieved. That invention discloses repeating the cycle each time 1.0 coulomb of capacity is removed from the battery and includes a counter circuit for counting the number of charge/discharge cycles of the capacitor producing a count representative of the amount of energy dissipated and therefore indirectly of the amount of energy remaining in the battery system.
It is well documented and accepted that the available energy in a battery is a function of the conditions to which the battery has been subjected. Capacity remaining is a complex function of current drain, temperature and time. Therefore, a reliable device for predicting remaining capacity has been actively sought and this need has been partially addressed in the above-referenced related patent disclosing a discharge level monitor for a battery. However, that patent's device neither addressed the need for a continuous internal means of determining capacity, which is desirable, nor provided the user the capability of resetting the monitor following battery recharging.
The above-referenced related patent's device provides an inexpensive, accurate state-of-charge indicators for high power, high energy batteries, particularly the lithium battery, but it does not address the need for providing a continuous internal means of determining battery capacity and that device is not being capable of being reset once full battery capacity is restored. Accurate state-of-charge indication can only be achieved by taking the discharge rate and temperature into account. The device provided in the above-referenced related patent could be efficiently combined with mechanisms to provide both continuous internal monitoring and resettability, and still be inexpensive, simply-implemented and accurate. The present invention discloses and claims such an improved battery state-of-charge indicator.